Learn English – Word for one nation taking the culture of another

Monocultural is what you want. One of the meanings of the noun monoculture is, in the OED’s definition, ‘A common culture or way of life; (now) especially a culture or way of life which is ascendant over a large (or global) area; a dominant cultural hegemony’ and it has been in use since 1968. The corresponding adjective monocultural is first recorded by the OED four years earlier.

The OED has no entry for unicultural.

In a very important sense, "culture" does include language. Your point, however, is that you want to emphasize both the common social elements of the culture in question, as well as the language of choice. So it's almost as if you are looking for something that goes against your actual goal. I might suggest that you stop trying to mash these things together, and simply look for different ways of saying it each time it comes up. Also pay attention to the fact that some of the times you are mentioning this fellow's interests might be unnecessary or superfluous, and so you may be able to cut down the number of times you have to say it.